If your System is over sized the air is colder. system runs a Quicker or cooling the air too fast not removing Humidity. Higher Humidity turns to Mold. The Air conditioner units cool the home and set the humidity level never turn off a vent in a room if your not in it. This make the Out Door heat and humidity enter the room. If you have return Air Vent in this room your pulling in hot air making system run longer.(I CAN BEVEIVE MONEY PIT SAID TURN OFF VENT IN ROOM'S YOU DO NOT USED)this IS INFO FROM 1970'S. And is not true. I keep my temp at 74-75 all the time if your not home the system runs shorter time due to no one opening doors. Now if a system is over charged this make evap., coil to cold too making mold in duct work. To Large system and under sized return in 99% of home's.